The lake Baikal seal: an evolutive biogeographic mystery ?

It is the “pearl of Siberia”, so named for its beauty and nature. As shown on the map below, it is located in the south of central Siberia, in the Russian Federation, quite near of Mongolia and China. Historically, the chinese called it as the North Sea. The Russians do not began to explore the lake until the end of 17th century. Although today most of the population is of russian origin, the south of the lake is inhabited by buryats, of Mongolian origin. They are the largest ethnic minority group in Siberia, with their own language. They are about 400,000 and their principal city is Ulan-Ude.

The famous Trans-Siberian railway passes beside the lake, bordering it by the southwest corner, with stops in the cities of Irkutsk and Ulan-Ude. This section is spectacular, with 200 bridges and 33 tunnels. This area is “only” about 5000 km far from Moscow (3 days) and 4000 km of Vladivostok (3 days) in the Pacific.

As you can see in this map, the lake is shaped like a crescent. It is the largest freshwater lake in the world (31,000 km2, like Catalonia or Belgium) and the deepest (1600 m maximum depth), and the world’s oldest (about 25-30 million years ago). The lake is in a rift, where tectonic plates are separating, and so it widens gradually.

It has an enormous biodiversity, with over 1700 species of plants and animals, 2/3 of which are endemic, and in 1996 UNESCO declared it a World Heritage Site. With minimum temperatures of -20°C most of the winter, lake Baikal is frozen for half the year.

The Baikal seal

The Baikal seal or “nerpa” is Pusa sibirica (formerly classified as Phoca sibirina), which is across all the lake, but nowhere else. The lake is 2400 km far from the Arctic ocean where the lake waters flow through the siberian river Yenissei.

The nerpa is one of the few species of freshwater seals. It is relatively small, measuring just over 1 meter and weighs about 60-70 kg. It eats several species of fish, but especially a so-called “golomyanka”, which is endemic of lake Baikal. This fish is very abundant in the lake, it is translucent, very fat (it is known as “oilfish”) and lives at depths of 200-500 meters. Seals dive down to eat them, they can resist up to 40 minutes underwater.

Well, it is not very clear, it is almost a mystery, since the seal lives far from the open seas, where all other species of seals live. But anyway, as for everything, we can look for the more reliable scientific hypothesis. Let’s see what we can found ….

This species is most similar to the Arctic seal or ringed seal, Pusa hispida, which lives in the Arctic. Like this and other of the family of Phocids (such as the common seal, Phoca vitulina), it is earless, unlike other Pinnipedia (sea lions and fur seals). Another species quite similar is the Caspian seal, Pusa caspica, which is also a curious case, because the Caspian is also an isolated sea. But the Caspian is almost a real sea, with a salinity of 1.2% (the third of the others seas and oceans), and therefore the Caspian seal, such as the Arctic, is not a freshwater seal.

However, there are two subspecies of the arctic seal, with few numbers, which are freshwater, like that of Baikal lake: they are the seals of lakes Saimaa (Finland) and Ladoga (Russia, near Finland), which are relatively near of the Arctic ocean. These two subspecies probably came from the Arctic, after the last glaciation, and they remained confined to these lakes. Probably, to change from salt water to freshwater is a relatively easy adaptation for mammals like these.

Another common feature of the Baikal seal and these of the same genus Pusa (the arctic and the Caspian ones), and others, is that their pups have white fur, changing it shortly after to the grey fur typical of adults. This suggests the origin of the common ancestor in a icy place, the arctic or a related environment.

Therefore, the prevailing scientific theory by 1960 [2] was that the origin of Baikal and Caspian seals from the Arctic would have been during one of the glacial periods of the Pleistocene, perhaps around 90,000 years ago, that is relatively recent in terms of evolution. As shown in the map below, during this period, north ice of the arctic covered part of Siberia (red line) and functioned as a barrier to all waters coming from the south, which currently drain to the Arctic ocean (by the rivers Ob and Yenissei, and others). Thus, a large lake was formed, which communicated probably with the Baikal and the Caspian, and all water flowed westward towards the Black Sea, as shown. Geologically, these connections of the Aral and Caspian to the Black Sea seem demonstrated [3, 4]. With this, the seals of the Arctic would have migrated south and led to the Caspian, and the Baikal perhaps.

Map collage of this from Mangerud [4] (left) with the one of freeworldmaps.net (right). The area in white until the red line was covered by north ice sheet, by 90,000 years ago.

But, as I mentioned, 90,000 years is a very short evolutionary time to explain the differences between these species, although they are closely related.

Therefore, it seems likely a previous common origin, and to prove it, molecular tools have been used the recent years. In this way, specific gene sequences of 12 mitochondrial proteins from different species have been compared. This study has been done including all pinnipeds [1].

Thus, in developing the corresponding dendrogram, as we see below, it can be observed that the evolutionary separation of the Baikal seal from the Arctic (ringed seal), and the Caspian, and the grey seal, occurred about 5 million years ago, at the beginning of the Pliocene. Therefore, these species of seals must have a common geographical origin, relating to these arctic lakes or internal seas. Molecular similarities have been also found between arctic amphipods (small crustaceans) and those of the Caspian Sea.

Dendrogram of phylogenetic relationship obtained for pinnipeds, according Arnason [1].

Furthermore, at the period when this seal speciation took place, and from the Oligocene (20 million years) to the Pliocene, the sea Paratethys (see diagram below) was extending from central Europe to this part Asia, over the Alps, Carpathians and other mountain ranges separating Paratethys from the Tethys sea. In fact, the current Black, Caspian and Aral seas, and other lakes in central Asia, are relics of what was the Paratethys. This great sea had connections with the Arctic during different periods.

Therefore, it is likely that the Caspian seal was originated from the Arctic ones or backwards, through these geographical connections of the Paratethys until the Pliocene. However, for our protagonist, the Baikal seal, there is not enough evidence to say that the Baikal was connected to the other seas of Paratethys because it is quite to the east. So, either Pusa sibirica came across this alleged connection with Paratethys, or maybe there was a separate settlement from the Arctic across the river Yenissei, also at this period. Nothing is discarded. Therefore, the Baikal seal will keep some mystery ……

References

[1] Arnason U. et al (2006) Pinniped phylogeny and a new hypothesis for their origin and dispersal. Molecular Phylogenetics and Evolution 41, 345-354

[2] McLaren I.A. (1960) On the origin of the Caspian and Baikal seals and the paleoclimatological implication. American Journal of Science 258, 47-64